nginx: ngx_msec_t is 32bit in 32bit platform
nginx: ngx_msec_t is 32bit in 32bit platform

`ngx_msec_t` is `uint_t` and it's 32bit in 32bit platform. However
`quiche_conn_timeout_as_millis()` returns `uint64_t` and comparing
`ngx_msec_t` with `UINT64_MAX` is not meaningful. (compiler
gives a warning)

Fix here is we check the return value of `quiche_conn_timeout_as_millis()`
first and cast to `ngx_msec_t`. In 64bit platform `ngx_msec_t` is
`uint64_t`, so no casting there.

Fixes #648.
cloudflare/quiche,cloudflare/quiche,cloudflare/quiche